TOOTH DEVELOPMENT — DASYPUS NOVEMCINCTUS 659 



which exactly resembles the compact enamel organ that I have 

 shown in figure 5. In my specimens there is certainly no stellate 

 reticulum represented in this mass of cells, but it is composed of 

 outer enamel epithelium, stratum intermedium, and inner enamel 

 epithelium. I have applied the term enamel to the secretion 

 deposited bj^ the ameloblasts, for, as far as I have been able to 

 prove, this secretion is true enamel. Spurgin also says that true 

 enamel is deposited upon these teeth, although I do not know 

 what criteria he used to determine that it is enamel. The secre- 

 tion which I have termed enamel is certainly deposited through 

 the agency of Tomes' processes, and its appearance is exactly 

 like that of the enamel on the developing tooth of the cat. I 

 have never been able to demonstrate satisfactorily the presence 

 of enamel prisms, but this is probably due to the fact that cal- 

 cification is not yet complete and also to the action of the decal- 

 cifying agent. However, in favorable specimens, as shown in 

 figure 4, the enamel is sometimes seen to be composed of darker 

 areas which have separated from each other, and between which 

 lighter areas appear. I have interpreted these darker areas as 

 representing the enamel prisms. Any breaks occurring in the 

 enamel are always in the direction that would be taken by enamel 

 prisms. Decalcification destroys all prismatic structure, so that 

 examination under a polarizing microscope is useless. Attempts 

 to grind down the calcified teeth have so far been unsuccessful 

 because of the extreme thinness of the enamel coat, which causes 

 it to break off in the process of grinding. The difficulty of deter- 

 mining the presence of enamel has already been recognized by 

 Tomes, who says (p. 30), "Although it might appear as an ex- 

 ceedingly simple matter to determine whether or not a tooth is 

 coated with enamel, as a matter of fact in practice it is not always 

 easy to be certain upon this point." Therefore, since I have no 

 evidence against the conclusion that the substance is enamel, 

 and since it is secreted through the agency of Tomes' processes, 

 and looks like the newly formed enamel in the cat's tooth, I feel 

 that I am justified in applying the term enamel to the secretion 

 which covers the dentine in the teeth of the armadillo. 



